Flash Photography

Electronic Flash
Flash Synchronization
Cameras with Focal-Plane Shutters
Cameras with Between-the-Lens (Leaf) Shutters
Dedicated Flash Sync

Flash Exposure
Exposure with an Automatic Flash Unit
Exposure with a Built in Flash
Exposure With A Dedicated Electronic Flash
Exposure with a Manual Flash Unit
Taking the Picture with a Manual Flash
Taking the Picture with an Automatic Flash
Taking the Picture with a Dedicated Flash
Shutter Speed for Flash Pictures

Variety in Flash Lighting
Bounce Flash
Off-Camera Flash
Fill-In Flash
Fill-Flash With a Manual Unit
Fill-Flash With an Automatic Unit
Preventing Flash Failure

Causes of Light Loss with Electronic Flash
Recycling Time
Weak Batteries
De-Forming of Condensers


With your 35 mm camera and an accessory (or built-in) electronic flash unit, you can take flash pictures indoors just about as easily as you take pictures in daylight. And as you'll see, you can also use your flash to improve the quality of your outdoor pictures, as well as to create special effects both indoors and out.

Electronic Flash

Although built-in flash units are now common on SLR camera, the extra power of an accessory flash gives you more options in setting f/stops and using different flash techniques. 

An electronic flash unit is convenient to use because it allows you to take a little slice of daylight with you wherever you go. A single set of batteries will provide hundreds of exposures. Also, because the light from electronic flash is similar to daylight, you don't have to worry about color balance as long as you're using daylight films. As an added bonus, the burst of light from electronic flash is brief enough to halt almost any subject or camera motion.

There are three basic types of electronic flash units: manual, automatic, and dedicated. These three types of flash units differ from one another mainly in the way that they (or you) determine exposure. With manual flash units, you determine the proper lens opening for your camera based on the guide number for your flash unit and the film you're using, or from a calculator dial on the flash. Manual units are slower to use because each time you change flash-to-subject distance, you must also change your lens aperture.

Automatic flash units have a light sensor that measures the light reflected by the subject from the flash and automatically controls the duration of the flash to produce the correct exposure.

You determine the aperture by using a calculator dial on the flash unit. Within a given distance range, the flash unit will provide accurate exposure even if you change your flash-to-subject distance.

Dedicated flash units, the most complex technologically, are the easiest to use. They also offer the most flexibility. Dedicated units automatically set your camera to the correct sync shutter speed and lens aperture, and then control exposure by regulating the amount of light the flash emits. These flash units measure the light with sensor on the unit or through the lens (TTL) by using the camera metering system. Many TTL units read the amount of light reflected off the film plane (called OTF flash) and automatically control the flash duration.

Flash Synchronization

The camera shutter speed that you use with a manual or automatic flash unit is very important. While the duration of the flash is extremely brief (usually measured in thousandths of a second), the burst of light must occur when the shutter is fully open; otherwise the shutter curtain may obscure part of the image. This timing between electronic flash and shutter is called flash synchronization or X sync.

Cameras with leaf shutters sync at all shutter speeds. Cameras with focal-plane shutters (nearly all SLR cameras) sync only at certain speeds. Although your main concern with shutter speed is synchronization, different shutter speeds can affect the appearance and exposure of your flash pictures in other ways.

Cameras with Focal-Plane Shutters

Almost all modern 35mm SLR cameras have focal-plane shutters. The typical synchronized shutter speed is 1/60, 1/125, or 1/250 second. If your camera has a shutter-speed dial, this speed is usually marked in red on the dial. If you have an older camera that has a switch with a choice of X sync or M sync (M sync is used for flashbulbs) or a choice of X or M-sync-cord sockets, use the X-sync setting or socket with electronic flash. 

The focal-plane shutter will be fully open to expose the film only at the specified shutter speed or slower shutter speeds. At faster speeds, the shutter curtain forms a moving slit. If you set a faster shutter speed, light from the flash will expose only the band of film uncovered by the slit at the moment the flash fires, and most of the scene will be cut off. One camera manufacturer has gotten around the sync problem with focal-plane shutters by modifying the flash unit to emit light during the full time the shutter (at any speed) is moving. 

Putting your focal-plane camera and flash in sync is simple: If your camera is equipped with a hot shoe*, all you have to do is attach the flash and set the shutter speed dial to the proper sync speed. If your camera doesn't have a hot shoe or if you're using the flash off-camera, you can use a sync cord to plug the flash into the X-sync socket. 

*A bracket on top of the camera to hold the flash. It's "hot" (nearly all are) if it has an electrical contact to complete a circuit with the flash unit. 

Cameras with Between-the-Lens (Leaf) Shutters

Some older cameras and many newer compact 35mm cameras have shutters located between the elements of the lens. Cameras with this type of shutter will synchronize with electronic flash at virtually any shutter speed. The ability to synchronize flash at any shutter speed is particularly useful for controlling very bright or very dim ambient light or for balancing flash and available light-for example, when you use fill flash. 

Dedicated Flash Sync

With dedicated flash units and compatible cameras, the camera will automatically set the sync shutter speed when you attach the flash unit. Although the camera may be able to sync at several shutter speeds, most dedicated flash/camera combinations will automatically choose the fastest possible shutter speed so that light from the flash, and not the ambient light, exposes the film. When you want to record some ambient light, you can usually manually set any shutter speed in the sync range.

Flash Exposure

Exposure with an Automatic Flash Unit

An automatic unit uses a calculator dial to indicate the aperture that will give the correct exposure for the speed of the film and the flash-to-subject distance. Once you have set the aperture, a light sensor in the flash unit will automatically adjust the duration of the flash to provide correct exposure within a specified distance range, for example 3 to 15 feet. As long as you stay within that range, your pictures should be correctly exposed. 

Some automatic units have a mode switch that enables you to choose from several apertures over different distances ranges. For example, the yellow mode may allow you to use f/5.6 over a distance range of 3 to 20 feet. A blue mode may allow you to use f/8 (for greater depth of field) over a distance range of 3 to 15 feet. Automatic units usually have a manual setting that lets you choose the best aperture for correct exposure in situations that might fool the automatic exposure system. Scenes that have large bright or dark areas, such as a white or dark brown wall, can trick the flash sensor. With large white areas, the automatic flash would underexpose (make the scene too dark), and with large dark areas, it would overexpose (make the scene too light). By setting the flash on manual, you disengage the sensor. But now you have to refer to the calculator dial and use the specific f-stop indicated for the flash-to-subject distance. As long as you are in the manual mode, you have to change the f-stop anytime you change the flash-to-subject distance. 

With light and dark scenes, you could also remain in the automatic mode if you simply used an f-stop one stop larger for light scenes (f/5.6 if f/8 indicated) and one stop smaller for dark scenes (f/11 if f/8 indicated). The exposure should be correct as long as you remain within the recommended range for the mode you're using. 

Exposure with a Built in Flash

Many compact and some advanced SLR cameras have small electronic flash units built into them. In some cameras, the flash is hidden (in the prism housing, for instance) and pops up when activated; in others the flash is built into the face of the camera. Although quite convenient, these low power flash units produce good results only within a rather short range (usually about 5 to 12 feet). You can use them as a main flash indoors or as a fill light outdoors. 

Some cameras automatically turn on the flash in dim light. Other cameras flash an indicator or beep to tell you to switch on the flash. Typically, the flash begins charging as soon as its activated, and on most models, an indicator light in or near the viewfinder will tell you when the flash is fully charged. With all built-in-flash cameras, exposure is fully automatic-measured either by an external sensor or inside the camera by a TTL or OTF metering system. 

What do you do in dimly lit situations where you don't want the flash to fire automatically? One way to keep some pop-up flash units from firing is to hold your finger on top of it to prevent it from popping up. Read your manual first to make sure that this won't damage your camera. With face-mounted built-in flash units, you may be able to block the flash by holding your fingers (or a small piece of black cardboard) in front of the flash. 

Cameras with built-in flash are particularly suited to outdoor fill flash work, and many are programmed to create an accurate exposure balance between flash and ambient light automatically. Some of these cameras will turn on the flash and automatically activate the fill flash whenever the foreground subject is significantly darker than the background. 

In addition to supplying a ready source of light, built-in flashes on some cameras provide other helpful services. On autofocus cameras, the built-in flash may have an autofocus illuminator to help your lenses focus in low light. With SLR cameras that have both built-in flash and the capability of accepting an accessory flash unit, you may be able to use the built-in flash as part of a multi-flash setup. To take a group portrait, for example, you might use an accessory flash bounced off the ceiling as your main light, and use the built-in flash as a frontal fill light to open up shadows in faces. 

Exposure With A Dedicated Electronic Flash

Dedicated electronic flash units provide the most versatile flash available. The term dedicated comes from the fact that these units are brand-specific; that is, they are designed to be used with or are dedicated to a certain brand (and often a specific model) of camera. As mentioned earlier, many sophisticated dedicated models measure the intensity of the flash inside the camera as it reflects off the film (OTF) itself; a few simpler models measure light from a flash-mounted sensor. 

While most dedicated flash units can be used as automatic (if they have a flash-mounted sensor) or as manual units with any other camera brand or model, they are truly dedicated only with a specific model of camera. Most dedicated equipment is made by camera manufacturers for their own cameras. Some universal dedicated flash units can be adapted to a variety of cameras with accessory modules, but most offer fewer features than a same brand flash. Read your camera manual and talk to your photo dealer before attaching any dedicated flash other than one designated to be used with your camera. 

Though they can handle complex tasks, dedicated flash units are extremely simple to use, because they'll make all the exposure decisions for you. Once you attach a dedicated flash to your camera, you are ready to start taking flash pictures. No calculations or complex tables are required. Through a series of electronic contacts and circuits, the flash and camera are able to relay information to one another almost instantaneously. 

After you mount the flash on the camera and turn them both on, the camera tells the flash the speed of the film in the camera, and automatically sets the proper shutter speed for flash sync. If you're using a camera with a programmed exposure mode, the camera also sets the lens aperture automatically. After you press the shutter button, the flash fires and the camera measures illumination at the film plane; when the film has received enough light for proper exposure, the camera turns the flash off. All of this happens in microseconds after you press the shutter button. 

With most dedicated flash/camera combinations, the settings chosen by the camera are displayed in the viewfinder, and sometimes on an LCD panel on the back of the flash. If you want a different aperture (to control depth of field, for example) or a slower shutter speed (to record a dimly lit background), you can usually switch to an aperture or shutter priority flash mode. If proper flash exposure with the aperture/shutter speed combination you have chosen is not possible, the flash and/or viewfinder will display a warning-usually by flashing the aperture or shutter speed or both. Some cameras with matrix-light metering will know if the background is unusually dim (at twilight, for instance) or bright (a white-sand beach) and set the exposure accordingly-using camera controls to expose for the background and flash duration to light the main subject. 

Other helpful displays in the viewfinder of a dedicated camera are a flash "ready" light to tell you that the flash is fully charged, and a sufficient-light indicator to tell you that the flash has provided adequate lighting. Units with an LCD panel may also provide you with a maximum distance (or the near/far range) for the aperture you select. This is a particularly useful feature, since it tells you beforehand if the aperture you're using will provide sufficient light at your working distance. 

Another benefit of using an OTF dedicated flash is that the amount of light is being measured behind the lens; this means that you don't have to make any special calculations when you use extension tubes or bellows for close-up work, or place colored filters over the flash or camera lens. And since dedicated cameras measure light at or near the film plane, you can use the flash in a bounce or off-camera mode and still get good exposure automatically. 

Dedicated flash units for some autofocus cameras have autofocus "illuminators" that enable the camera to focus in very dim or even totally dark situations. By projecting a visible near infrared grid of red light on the subject (usually when the shutter button is partially depressed), the flash provides the camera with sufficient contrast for accurate focusing. In any flash situation, the camera may adjust flash power or aperture based on distance information provided by an autofocus lens. 

The options and possibilities offered by dedicated flash are many and useful; take the time to study your manual fully to get the best results from your flash. 

Exposure with a Manual Flash Unit

Even though only a few manual flash units are being sold today, the ability to figure exposure with a manual flash unit (or an auto or dedicated unit in the manual mode) can be very useful. In extreme close-up photography and outdoor flash fill photography, for example, you may be faced with situations in which manual flash exposure would provide more accurate and predictable results. The same is true of subjects that might fool an auto-flash sensor-such as a particularly dark or light subject. 

To determine the correct lens opening for proper exposure with a manual flash unit, you need to know the light output of the electronic flash unit you're using, the speed of the film you're using, and your distance from your subject and how to calculate these factors. 

Most manual (and some auto) flash units have a dial that will perform calculations for you-simply set the speed of the film you're using and read the correct aperture opposite the flash-to-subject distance. If your flash unit doesn't have a calculator dial, you need to use a guide number for your calculations. A guide number lumps all these variables into one easy-to-use number for each combination of flash power and film speed. Guide numbers are easy to use and are usually provided in the flash manual. 

To figure the correct exposure with a manual flash unit, simply divide the guide number for your film/flash combination by the flash-to-subject distance in feet. (If your flash is camera mounted, you can simply read the subject distance from the focusing scale on the lens barrel.) The result is the lens opening to use. Here's how the formula looks:. 

Guide Number/Distance in Feet = Lens Opening 

For example, if the guide number is 110 and the subject is 10 feet away, the correct lens opening is f/11 When the calculated lens opening is one that's not marked on the f/number scale of your camera lens, just use the nearest one that is marked or a point halfway between the two nearest ones. Of course, if you have a metric guide number, you would divide by the distance in meters to obtain the f/number.

Taking the Picture with a Manual Flash

                 1. Mount the flash on the camera's hot shoe. Or if you
                     are using off-camera flash, connect the flash to the
                     camera with a sync cord.

                 2. Set the speed of your film on the flash calculator
                     dial. If the flash has no dial, look in your instruction
                     booklet for the proper guide number for the film
                     you're using.

                 3. If you are using an automatic or dedicated flash in
                     the manual mode, set the flash to "manual.".

                 4. Set the shutter speed at the fastest available sync
                     speed, usually 1/60 or 1/125 second. If you are
                     using an automatic programmed camera, also be
                     sure to set the camera on manual and then set the
                     shutter speed.

                 5. Focus on your main subject and read the distance
                     opposite the index mark on the lens barrel.

                 6. Find this distance on the flash calculator dial and
                     set your camera lens to the aperture indicated on the
                     dial. Again, if there is no dial, you can figure the
                     correct aperture by dividing the distance of your
                     subject into the flash guide number.

                 7. Turn the flash on and wait for the ready light to

                 8. Take the picture. Manual flash uses a lot of power,
                     so recharging will take longer than with an
                     automatic flash. However, if the flash takes an
                     unusually long time to charge between shots (more
                     than 12 to 15 seconds), install fresh batteries.

Taking the Picture with an Automatic Flash

                 1. Mount the flash on the camera's hot shoe, or connect
                     it to the camera with a sync cord if you're holding
                     the flash off-camera. 

                 2. Set the film speed on the flash calculator dial. 

                 3. Set the fastest available sync speed on the
                     shutter-speed dial, usually 1/60 or 1/125 second.
                     The fastest sync speed is usually marked in red on
                     the dial. If you are using an auto-exposure camera,
                     don't set the camera to automatic because the
                     camera and flash may be out of sync-set it to the
                     sync shutter speed. 

                 4. Focus on your main subject and read the distance
                     opposite the index mark on the lens barrel. 

                 5. Find this distance on the flash calculator dial and
                     set the lens to the aperture indicated on the dial.
                     Many auto-flash units offer a choice of several
                     aperture/mode combinations; at any given distance,
                     you may have a choice of four or more shooting
                     apertures. To find out which apertures you can use
                     at a given distance, see your flash manual. Each
                     different aperture/mode pairing is usually
                     color-coded to a mode-selector dial. 

                 6. Turn the flash on and wait for the ready light to

                 7. Take the picture. If your flash has a sufficient-light
                     indicator, check to see that it has lit. If it hasn't, be
                     sure that you are working within the distance range
                     that applies to that aperture. 

                     When you are using the flash in a bounce mode and
                     you don't get a sufficient-light indication, you may
                     have to use the next larger aperture/ mode
                     combination-even if you're within the correct
                     working distance. This is because the bounce
                     surface absorbs and scatters much of the light from
                     the flash. Or you can move closer to your subject.

Taking the Picture with a Dedicated Flash

                 1. Mount the flash on the camera's hot shoe.

                 2. Set the film speed on the camera and on the flash if
                     it has a film-speed dial. Some cameras will read
                     the DX code and set the film speed for both the
                     camera and the flash.

                 3. If you are working in the full program mode, set the
                     flash to the TTL (through-the-lens) or OTF
                     (off-the-film) mode.

                 4. Set the camera mode selector to "P" (program)
                     mode or the equivalent mode on your camera (see
                     your manual). The camera will automatically set the
                     proper sync speed and aperture. As you move
                     around a subject and the ambient light changes, the
                     camera may also change the aperture. On some
                     cameras, you may have to set the aperture ring to
                     the "A" position or the smallest aperture for
                     programmed operation.

                 5. If your camera has an LCD panel or printed scale
                     that tells you the working distance range for the
                     aperture it has chosen, be sure that you remain
                     within that distance, or check your instruction
                     booklet for flash-to-subject limits. If your dedicated
                     flash unit has the light sensor on the unit (non-TTL),
                     you may have to set the aperture yourself. To do
                     this, follow the instructions for choosing an
                     aperture for automatic flash.

                 6. Turn the flash on and wait for the ready light to

                 7. Take the picture.

                 8. Most dedicated cameras have a sufficient-light
                     indicator in the viewfinder that will light if the
                     subject has received adequate light. If it doesn't
                     light, check to see that you have the flash and
                     camera in the proper modes and that you are within
                     the required distance range.

Shutter Speed for Flash Pictures

Because the duration of electronic flash is so brief, the camera shutter stays open for a period longer than the flash duration-even leaf-type shutters at high shutter speeds. As a result, the shutter lets all of the light from the electronic flash pass through the lens regardless of shutter speed. Consequently, changing the shutter speed with electronic flash does not affect the exposure for a main subject illuminated mainly by flash. Different shutter speeds can, however, alter the appearance of the scene. In some situations you may want background light to register; in others, you will want to use shutter speed to subdue it. 

When you want to minimize the background light, use the fastest sync speed possible (such as 1/125 or 1/250 second). You might want to control background light in this way, for example, if you're using daylight film and the background lights are tungsten (giving an orange cast to the background). A fast shutter speed is also useful with fillflash. Use a fast shutter speed to get proper exposure for a bright background when you want to use a relatively large lens aperture-to use selective focus in an outdoor portrait, for instance. Also, if there is strong light present when you are photographing action with electronic flash, use a shutter speed fast enough to stop the action or you will get a "ghosting" effect from the moving subject. 

If stopping action is not a problem and if dimly lit background detail is important, use a slower shutter speed, such as 1/30 second. For example, if you are photographing a person on a beach at twilight, and want to use flash to illuminate your subject and still record the colors of the sky, a slow shutter speed would be the answer. As mentioned earlier, some auto and dedicated units will provide this background/subject balance automatically. If you use speeds slower than 1/30 second, remember to use a tripod.

Variety in Flash Lighting

The simplest and fastest way to take flash pictures is with the flash mounted on the camera. However, if you want to take flash pictures that appear more professional, you may want to try the special flash techniques that follow.

Bounce Flash

With this technique you aim the flash at a ceiling or wall and bounce the light back onto your subject. This produces gentle, even lighting similar to that found outdoors on an overcast day. Bounce flash is especially useful for large interiors or group portraits, because you can light a large area more subtly than with direct flash. With color film, be sure to aim the flash at a white or near-white ceiling or wall. Otherwise your subject may pick up a color cast from the reflecting surface. White is also an efficient reflector of light. 

Flash units designed for bounce flash can usually be tilted or swiveled to let you bounce flash off a ceiling for both vertical and horizontal pictures. Units with lateral swivel can also be aimed at a wall. If your flash has no built-in bounce capability, you can probably purchase a special bracket that will allow you to position the flash in any direction. 

When you use any camera-mounted automatic flash unit (or a dedicated flash with a flash-mounted sensor) for bounce lighting, the light sensor of the flash must always be aimed at your subject to determine exposure correctly. Exposure for bounce flash with a dedicated camera and flash combination that uses a TTL or OTF metering system is completely automatic and accurate in almost all situations, since the light is being measured inside the camera. 

Remember that even though your flash will figure bounce exposure automatically, just as it would for on-camera flash picture, the maximum flash-to-subject distance recommended by the flash manufacturer is intended for direct flash only. With bounce flash, you lose about 50-percent efficiency. To avoid underexposure with bounce flash, be sure that the flash-to-ceiling-to-subject distance is less than half the maximum recommended flash distance for the aperture you're using. Check the maximum distance on your flash calculator dial. 

If your automatic flash unit (or dedicated camera/flash) has a sufficient-light indicator, it will help you determine if your exposure for bounce flash is okay. With some units, you can test for sufficient light without actually taking a picture. To use this feature, you fire the flash without taking a picture and watch the indicator light to see if there is enough light from the flash. Most flash units have a button for firing the flash independently of the camera. If there's not enough light, you will have to use a larger lens opening, move closer to your subject, or use a faster film. 

If you are using an automatic flash unit that has no tilt or swivel capability, the only way to use it for bounce is to take it off the camera and aim the whole unit at your bounce surface. However, because the exposure sensor will be aimed in the same direction as the flash (at the bounce surface and not the subject), it will read the light reflecting from that surface and not the subject- producing the wrong exposure. The only solution is to switch the flash unit to manual. 

To determine exposure for bounce flash manually, first find the total distance from the flash to the bounce surface and from the bounce surface to your subject. Find the aperture that corresponds to this distance on the flash calculator dial; then set an aperture 1 1/2 to 2 stops larger than the aperture on the dial. For example, if the dial recommends f/8, use f/4. 

If the flash doesn't have a calculator dial, divide the guide number by the total distance. Then use a lens opening 1 1/2 to 2 stops larger than the f-number calculated from the guide number. Exposure also is affected by the size and color of the room. 

Off-Camera Flash

Although on-camera flash produces well-exposed pictures, its flat frontlighting makes subjects seem one-dimensional. Off-camera flash makes subjects look three-dimensional. It also creates interesting highlights and shadows on your subject. People will appear more realistic when facial texture and forms are revealed. By placing off-camera flash at a high angle, you can separate subjects from their surroundings and subdue distracting background shadows that head-on flash often causes. 

To use your flash off the camera, you'll need a long flash synchronization cord (also called a "PC" cord). Now you can hold the flash in one hand and the camera in the other. Don't worry if you're slightly wobbly in holding the camera. The flash will freeze camera motion. You can also mount the flash on a light stand or have a helper hold it. Make sure to set the aperture for the flash-to-subject distance and to aim the flash sensor at the subject. A special sync cord with a remote auto-sensor that fits in the camera's hot shoe is available for some flash units. Once aimed at the subject, the flash sensor will provide correct exposure no matter where you hold or aim the flash. 

With a dedicated flash unit, you will need to use an extension cord specifically dedicated to your camera/flash combination that attaches to the camera's hot shoe. This cord carries information between camera and flash so that both units can carry on their electronic conversation. Dedicated flash/camera combinations with TTL/OTF metering are especially accurate for off-camera use because the light is always being measured at the film plane. 

Of course, you can also use a manual flash off camera (or an automatic or dedicated flash set to manual), but you will have to calculate your exposure differently. Instead of dividing the guide number by the distance from the camera to the subject (as you would with a camera-mounted flash) to get your working aperture, you must divide the guide number by the distance from the flash to the subject. You may have to adjust the aperture slightly for very dark or light subjects. 

Fill-In Flash

When subjects are in bright sunlight, deep shadows often obscure important details. You can lighten these shadows to reveal detail by using your flash. This is called fill flash. 

Fill flash is particularly useful for lightening shadows on faces. As a bonus, when you're using fill flash with people, you can turn your subjects away from the sun so  that they don't have to squint. The rim lighting created by the sun in backlit portraits also gives a pretty glow to hair and helps separate your subjects from distracting surroundings. 

For best results, use a lower medium-speed film so that the camera can provide a good balance between sunlight and flash exposure. 100 and 200 ISO films are all good choices. 

The ideal fill-flash exposure gives your subject about one stop less exposure than the background receives. This opens the shadows sufficiently to reveal detail as it maintains a natural appearance. If you give the main subject the same exposure as the background, the picture tends to look artificially lit. 

You can create fill flash with dedicated, automatic, or manual flash units. An automatic flash unit will be much easier to use, however, if you have a choice of several flash modes to choose from. 

By far the simplest method of creating flash fill is with a dedicated flash/camera combination that has automatic fill-flash capability. The advent of dedicated flash/camera combinations with TTL or OTF metering, particularly those cameras with matrix OTF metering systems, has all but eliminated the need to calculate exposure for fill-flash pictures. These systems are programmed to provide proper exposure for the background and automatically fill the main subject with about one stop less light. A few sophisticated dedicated SLR camera/flash combinations even allow you to manipulate this subject-to-background lighting ratio with a switch. 

In any case, using a programmed camera with OTF metering and automatic fill may require no more than attaching the flash to the camera. The camera will measure the ambient light and supply the correct amount of flash to fill your main subject. If the camera also has autofocus, it may even use the distance information from the lens to help calculate the amount of fill required. As you move closer or farther from the subject, the camera will adjust the aperture and/or the amount of fill accordingly. Also, if you want to maintain a constant aperture (e.g., for depth-of-field), some cameras let you use an aperture-priority mode and still get correct fill flash. 

Review your equipment manuals to learn how to use fill-flash with your camera and flash. 

One caution: If you are using a compact or SLR camera with a built-in flash, be aware that cameras that offer "automatic flash" may not offer "automatic fill flash."

The difference is that the former will provide adequate flash exposure for the subject, but it will provide equal exposure for subject and background. Cameras that offer automatic fill will automatically create a more natural-looking brightness relationship between subject and background. Still, using a camera with automatic flash in contrasty situations is better than using no flash at all. 

Calculating correct exposure with an automatic or a manual flash unit (or an automatic used in the manual mode) is a little more time-consuming, but can be  reduced to a series of easy-to-follow steps. With either type of flash, the basic principle is to set the lens aperture for the ambient light and then use the flash to create light that is one-half to one-fourth as bright as the ambient light. 

First take a meter reading of the bright ambient light. To do this, set the camera shutter-speed dial to the fastest sync speed available, and then set the lens aperture as the meter indicates. In the manual mode, after making your initial reading of the ambient light, you can control the amount of fill light by altering the flash-to-subject distance. With an automatic flash unit, you manipulate the amount of fill flash by choosing a mode that matches the amount of fill you want to use. 

Fill-Flash With a Manual Unit

              1. Mount the flash on the camera's hot shoe, and if you
                  are using an automatic flash, set the unit on manual. 

              2. Set the film speed on the flash calculator dial. 

              3. Set the fastest sync shutter speed. 

              4. Take a meter reading of the sunlit area and set the
                  lens at the indicated aperture. 

              5. On the flash dial find the aperture that is 1 stop
                  larger than the meter reading indicated in step 4 (for
                  example, if you set the lens at f/11, find f/8 on the
                  calculator dial). Read the distance indicated
                  opposite this aperture. This is the correct
                  flash-to-subject distance for fill flash that is 1 stop
                  less bright than the sunlit area (3:1 lighting ratio).
                  Move to this distance to shoot your picture (or take
                  the flash off camera and move it to that distance). If
                  you want a stronger fill (equal to the ambient-light),
                  you can simply shoot from the distance opposite the
                  f-stop that you set on your lens. For a weaker fill,
                  move the flash farther away from the subject. Use a
                  zoom lens to adjust image size once you are at the
                  proper distance. 

              6. Turn on the flash unit, and shoot the picture when
                  the ready light glows.

Fill-Flash With an Automatic Unit

              1. Mount the flash on the camera's hot shoe and set the
                  film speed on the flash calculator dial. 

              2. Set the fastest sync shutter speed. 

              3. Take a meter reading of the sunlit area and set the
                  lens to the indicated aperture.

              4. Set the flash mode switch. Look at the flash
                  calculator dial and find the mode corresponding to
                  the aperture 1 stop larger than that set on the lens
                  (for example, if f/11 is set on the lens, set the mode
                  that corresponds to f/8). This will give you a
                  pleasing 3:1 lighting ratio. You could also make the
                  fill 2 stops dimmer (5:1 lighting ratio) than the
                  ambient-light by using a mode that requires 2 more
                  stops than the aperture set on the lens (f/5.6 if lens
                  is set to f/11). If there is no flash mode to match the
                  aperture your meter has chosen, switch your shutter
                  speed/aperture combination to one that uses a
                  shutter speed that's 1 speed slower; then look to see
                  if there is a mode that matches the corresponding
                  new aperture. This will give you the same
                  ambient-light exposure, but will allow you to work
                  within the allowable aperture range for the flash. 

              5. Turn on the flash unit and shoot the picture when the
                  ready light glows.

NOTE: The light sensor on most automatic flashes will be affected only by the flash reflecting from your subject not by the ambient light. However, in situations where there is very bright light spilling around the edges of your subject and falling directly onto the flash, it may give the flash false readings. One answer is move your subject so that the excess light is not hitting the flash directly. With automatic-flash it's important to experiment with your flash in different situations and see how it performs; see your flash manual for specific instructions.

Preventing Flash Failure

The most frequent causes of flash failure are weak batteries and battery or equipment contacts that need cleaning. With electronic flash units, when the time required for the ready light to come on becomes excessive-about 30 seconds or longer-or it doesn't come on at all, it usually means the batteries are weak and need to be replaced or recharged, depending on the kind of batteries. See your flash equipment manual. 

If there are deposits on the equipment or battery contacts, even brand-new batteries won't fire the flash. To prevent this type of flash failure, clean the battery ends and equipment contacts with a rough cloth. If the battery compartment in the flash unit is small, wrap the cloth over the end of a pencil eraser to clean the contacts. Clean the contacts even if they look clean, because some deposits are invisible. 

Be sure to use the type and size of battery recommended for your flash equipment. Alkaline batteries have a long life and a short recuperation time. They are generally recommended for electronic flash units. Nickel-cadmium batteries are rechargeable and are recommended for use in many electronic flash units. See your flash instruction manual for the kind of battery recommended for your flash unit. 

When you're not going to use your flash unit for a period of time, remove the batteries to prevent possible corrosion of the contacts in the unit. 

Check the fittings between the flash and the camera to see that they remain tight. If your flash connects to your camera with a flash cord, make sure any press-on adapters are tight. Also, a break in the cord will prevent the flash from firing. You can often detect an internal break in the cord by wiggling the cord. When momentary contact is made, the flash will fire. When you detect such a break, replace the cord.

Causes of Light Loss with Electronic Flash

Recycling Time

After you have fired an electronic flash unit, it takes several seconds for the condensers in the unit to recharge. Most electronic flash units have a ready light that comes on after about 6 seconds, depending on the unit, to indicate that the unit is ready to flash. But at this point you may get only about 65 percent of the total light output because the ready light does not necessarily indicate when the condensers in the unit are fully charged. Recycling time for full light output varies in practice and depends on the electronic components in the unit, type and condition of batteries, and other factors. An automatic flash unit with an energy-saving circuit, called a thyristor circuit, will recharge more quickly on utomatic than a unit without this circuit. An AC-powered unit may recharge faster than a battery-powered unit. 

You will get more consistent photographic results if you wait until your flash unit has recycled completely before taking the next picture. After the ready light comes on, wait a few seconds before firing the flash. Or to be on the safe side, allow at least 30 seconds between flashes, because it may take that long for the condensers in some units to recharge fully. If necessary, you can take pictures more rapidly than this, but you may not get full light output from your flash unit. This can cause underexposed pictures, depending on the exposure latitude of the film you are using.

Weak Batteries

As the batteries in your flash unit lose power with use and age, the recycling time increases. When the battery power drops below the required level, the unit will lose light output even though it may still flash. Replace or recharge the batteries whenever the recycling time becomes excessive. Also, remember that it's important to keep the battery and flash contacts clean by wiping them with a rough cloth.

De-Forming of Condensers

Another factor which can weaken batteries and cause a loss of light output is the tendency of the condensers in an electronic flash unit to de-form after a month or so of inactivity. When this happens it will take an extra-long time to re-form the condensers and bring them back up to a full charge. This reforming puts a considerable drain on the batteries. If you can use regular house current (AC) to power your unit, re-form the condensers by letting them recharge from the power line for an hour or so-and fire the flash a half dozen times-whenever the unit has been out of use for a few weeks. This helps your flash unit produce full light output.

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